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CSS3 is the latest evolution of the Cascading Style Sheets language and aims at extending CSS2.1. It brings a lot of long-awaited novelties, like rounded corners, shadows, gradients , transitions or animations , as well as new layouts like multi-columns , flexible box or grid layouts. Experimental parts are vendor-prefixed and should either be avoided in production environments, or used with extreme caution as both their syntax and semantics can change in the future.

Modules and the standardization process

CSS Level 2 needed 9 years, from August 2002 to June 2011 to reach the Recommendation status. This was due to the fact that a few secondary features hold back the whole specification. In order to accelerate the standardization of non-problematic features, the CSS Working Group of the W3C, in a decision referred as the Beijing doctrine , divided CSS in smaller components called modules . Each of these modules is now an independent part of the language and moves towards standardization at its own pace. While some modules are already W3C Recommendations, other still are early Working Drafts. New modules are also added when new needs are identified.

Formally, there is no CSS3 standard per se . Each module being standardized independently, the standard CSS consists of CSS2.1 amended and extended by the completed modules, not necessary all with the same level number. At each point of time, a snapshot of the CSS standard can be defined, listing CSS2.1 and the mature modules.

The W3 consortium periodically publishes such snapshots, like in 2007 or 2010.

Though today no module with a level greater than 3 is standardized, this will change in the future. Some modules, like Selectors 4 or CSS Borders and Backgrounds Level 4 already have an Editor's Draft, though they haven't yet reached the First Published Working Draft status.

CSS modules status

Stable modules

A few CSS modules are already fairly stable and have reached one of the three recommendation level of the CSSWG: Candidate Recommendation, Proposed Recommendation or Recommendation. These can be used without prefixed and are pretty stable, though a few features can still be dropped at the Candidate Recommendation stage.

These modules extend and amend the CSS2.1 specification which build the core of the specification. Together with it, they are the current snapshot of the CSS specification.

{{ SpecName("CSS3 Colors", "", "") }}

{{ Spec2("CSS3 Colors") }} since June 7th, 2011

Adds the {{ cssxref("opacity") }} property, and the hsl(), hsla(), rgba() and rgb() functions to create {{ xref_csscolorvalue() }} values. It also defines the currentColor keyword as a valid color.

The transparent color is now a real color (thanks to the support for the alpha channel) and is a now an alias for rgba(0,0,0,0.0) .

Media queries are not only used in CSS document but also in some attributes of HTML Elements, like the {{ htmlattrxref("media","link") }} attribute of the {{ HTMLElement("link") }} element.

The next iteration of this specification is in the work, allowing to tailor a Web site regarding the input methods available on the user agent, with new media features like hover or pointer. Detection of EcmaScript support, using the script media features is also proposed.

{{ SpecName("CSS3 Style", "", "") }}

{{ Spec2("CSS3 Style") }} since November 7th, 2013

Formally defines the syntax of the content of the HTML style global attribute.

{{ SpecName("CSS3 Backgrounds", "", "") }}

{{ Spec2("CSS3 Backgrounds") }}

Adds:

Support, on backgrounds, for any type of {{ xref_cssimage() }}, and not only for uri() defined ones.

Support for multiple background images.

The {{ cssxref("background-repeat") }} space and round values, and for the 2-value syntax of this CSS property.

Support for the use of an {{ xref_cssimage() }} as the border with the CSS {{ cssxref("border-image") }}, {{ cssxref("border-image-source") }}, {{ cssxref("border-image-slice") }}, {{ cssxref("border-image-width") }}, {{ cssxref("border-image-outset") }}, and {{ cssxref("border-image-repeat") }} properties.

Support for shadows of the element with the CSS {{ cssxref("box-shadow") }} property.

The CSS4 iteration of the Backgrounds and Borders specification is already in progress, though it still hasn't reached the First Public Working Draft stage, it plans to add the ability to clip a border (with the CSS {{ cssxref("border-clip") }}, {{ cssxref("border-clip-top") }}, {{ cssxref("border-clip-right") }}, {{ cssxref("border-clip-bottom") }}, and {{ cssxref("border-clip-left") }} properties) or to control the shape of the border in a corner (using the CSS {{ cssxref("border-corner-shape") }} property).

Extends the url() syntax to support image slices using media fragments.

Adds:

The dppx unit to the {{ xref_cssresolution() }} data type.

The image() function as a more flexible alternative to url() to define an image from an url.At risk: due to insufficient browser support, standardization of the image() function may be postponed to the next iteration of this module.

Support for linear-gradient(), repeating-linear-gradient(), radial-gradient() and repeating-radial-gradient().

The ability to define how a replaced element should fit in its element, using the CSS {{ cssxref("object-fit") }} property.At risk: due to insufficient browser support, standardization of the {{ cssxref("object-fit") }} and property may be postponed to the next iteration of this module.

The ability to override the resolution and orientation of an external image using the CSS {{ cssxref("image-resolution") }} and {{ cssxref("image-orientation") }} properties.At risk: due to insufficient browser support, standardization of the {{ cssxref("image-resolution") }} and {{ cssxref("image-orientation") }} properties may be postponed to the next iteration of this module.

Definition for the {{ cssxref("calc", "calc()") }}, {{ cssxref("attr", "attr()")}}, and toggle() functional notations.At risk: due to insufficient browser support, standardization of the calc(), attr(), and toggle() functional notations may be postponed to the next iteration of this module.

Several types definition, like <ident> and <custom-ident>, have been deferred to CSS Values and Units Module Level 4.

Adds features for conditional processing of parts of style sheets, conditioned on capabilities of the browser or the document the style sheet is being applied to. It consists mainly in allowing nested at-rules inside {{ cssxref("@media") }} and the adding of a new CSS at-rule, {{ cssxref("@supports") }}, and a new DOM method {{domxref("CSS.supports()")}}.

{{ SpecName("CSS3 Text-decoration", "", "") }}

{{ Spec2("CSS3 Text-decoration") }}

Extends:

the CSS {{ cssxref("text-decoration") }} property by making it a shorthand for the CSS {{ cssxref("text-decoration-line") }}, {{ cssxref("text-decoration-color") }}, and {{ cssxref("text-decoration-style") }} properties. And adds the {{ cssxref("text-decoration-skip") }}, and {{ cssxref("text-underline-position") }} properties.

Support for script shadows with the CSS {{ cssxref("text-shadow") }} property.

Precises:

The paint order of the decorations.

At risk: due to insufficient browser support, standardization of the text-decoration-skip, line positioning rules and the ability to place both emphasis marks and ruby above the same base text may be postponed to the next iteration of this module.

{{ SpecName("CSS3 Fonts", "", "") }}

{{ Spec2("CSS3 Fonts") }}

Amends the CSS2.1 Font matching algorithm to be closer to what is really implemented.

The control of the automatic generation of an oblique or bold face when none are found via the CSS {{ cssxref("font-synthesis") }} property.

{{ SpecName("CSS3 Syntax", "", "") }}

{{ Spec2("CSS3 Syntax") }}

Precises how charsets are determined; minor changes in parsing and tokenization algorithms.

Modules in the refining phase

Specifications that are deemed to be in the refining phase are already fairly stable. Though changes are still expected, they shouldn't create incompatibilities with current implementations; they should mainly define behavior in edge cases.

{{ SpecName("CSS3 Basic UI", "", "") }}

{{ Spec2("CSS3 Basic UI") }}

Adds:

The ability to tweak the box model using the CSS {{ cssxref("box-sizing") }} property.At risk: due to insufficient browser support, standardization of the padding-box value may be postponed to the next iteration of this module.

Support for icons, defined by the CSS {{ cssxref("icon") }} property simultaneously with the new icon value of the CSS {{ cssxref("content") }} property.At risk: due to insufficient browser support, standardization of the {{ cssxref("icon") }} property and the icon value may be postponed to CSS4.

Support for the CSS {{ cssxref("outline-offset") }} property giving more control on the position of the outline.

Support for the CSS {{ cssxref("resize") }} property allowing Web authors to control if and how elements should be resized.

Support for the CSS {{ cssxref("text-overflow") }} property defining how text overflows, if needed.At risk: due to insufficient browser support, the 2-value syntax of this property as well as the support for {{ xref_cssstring() }} values may be postponed to the next iteration of this module.

The ability to define the hotspot of a cursor as well as the new none, context-menu, cell, vertical-text, alias, copy, no-drop, not-allowed, nesw-resize, nwse-resize, col-resize, row-resize, all-scroll, zoom-in, zoom-out, extending the {{ cssxref("cursor") }} property.

The ability to specify the sequential navigation order (that is the tabbing order ) using the CSS {{ cssxref("nav-index") }}, {{ cssxref("nav-up") }}, {{ cssxref("nav-right") }}, {{ cssxref("nav-left") }}, {{ cssxref("nav-down") }} properties.At risk: due to insufficient browser support, standardization of the navigation properties may be postponed to the next iteration of this module.

The ability to control the usage of an IME editor, using the CSS {{ cssxref("ime-mode") }} property.At risk: due to insufficient browser support, standardization of the {{ cssxref("ime-mode") }} property may be postponed to the next iteration of this module.

An early list of what could be in the next iteration of the CSS Basic User Interface Module is available.

the support of bi-dimensional transforms to be applied to any element using the CSS {{ cssxref("transform") }} and {{ cssxref("transform-origin") }} properties. The supported transforms are: matrix(), translate(), translateX(), translateY(), scale(), scaleX(), scaleY(), rotate(), skewX(), and skewY().

the support of tri-dimensional transforms to be applied to any element by adding the CSS {{ cssxref("transform-style") }}, {{ cssxref("perspective") }}, {{ cssxref("perspective-origin") }}, and {{ cssxref("backface-visibility") }} properties and extended the {{ cssxref("transform") }} property with the following transforms are: matrix3d(), translate3d(), translateZ(), scale3d(), scaleZ(), rotate3d(), rotateX() ,rotateY(), rotateZ(), and perspective().

Note: this specification is a merge of CSS 2D-Transforms, CSS 3D-Transforms and SVG transforms.

{{ SpecName("CSS3 Fragmentation", "", "") }}

{{ Spec2("CSS3 Fragmentation") }}

Defines how partitions of a Web page should happen, that is page, column breaks, and widows and orphans handling.

Adds:

Support for defining the behavior of decorations, that is borders and background colors or images, when a box is breaked (at a page, column or line-break) with the CSS {{ cssxref("box-decoration-break") }} property.

{{ SpecName("CSS3 Text", "", "") }}

{{ Spec2("CSS3 Text") }}

Extends:

the CSS {{ cssxref("text-transform") }} property with the value full-width.

the CSS {{ cssxref("text-align") }} property with the value start, end, start end, and match-parent for a better support of documents with multiple directionalities of text.

the CSS {{ cssxref("text-align") }} property with a {{ xref_cssstring() }} value to align on that character. This is useful to align number on the decimal point.

the CSS {{ cssxref("word-spacing") }} and {{ cssxref("letter-spacing") }} properties with range constraints to control flexibility in justification.

Adds:

Control on how whitespaces are displayed using the CSS {{ cssxref("text-space-collapse") }} and {{ cssxref("tab-size") }} properties.

Modules in the revising phase

Modules that are in the revising phase are much less stable than those in the refining phase. Often the syntax is still under scrutiny and may evolve a lot, in a non-compatible way. Alternative syntax are tested and often implemented.

{{ SpecName("CSS3 Writing Modes", "", "") }}

{{ Spec2("CSS3 Writing Modes") }}

Defines the writing modes of both horizontal and vertical scripts and precises how the CSS {{ cssxref("direction") }} and {{ cssxref("unicode-bidi") }} properties interact with the new CSS {{ cssxref("text-orientation") }} property, and extends them where needed.

Modules in the exploring phase

{{ SpecName("CSS4 Images", "", "") }}

{{ Spec2("CSS4 Images") }}

Extends:

the image() functional notation to describe the directionality of the image (rtl or ltr), allowing for bidi-sensitive images.

the {{ cssxref("image-orientation") }} property by adding the keyword from-image, allowing to follow EXIF data stored into images to be considered.

Adds:

the image-set() functional notation to allow the definition to equivalent images at different resolution allowing for resolution-negotiated selection of images.

the element() functional notation allowing the use of part of the page as image.

the cross-fade() functional notation allowing to refer to intermediate images when transitioning between two images and defines the interpolation between two images.

the conic-gradient() and repeating-conic-gradient() functional notation describing a new type of gradient.

the {{cssxref("image-rendering")}} property that allow to define how resize of the object should be handled.

{{ SpecName("CSS3 Device", "", "") }}

{{ Spec2("CSS3 Device") }}

Adds a new at-rule, {{ cssxref("@viewport") }}, allowing to specify the size, zoom factor, and orientation of the viewport that is used as the base for the initial containing block.